1. Field of the Invention
The present invention relates to a non-aqueous electrolyte solution and a lithium secondary battery using the same.
2. Description of the Related Art
Conventionally, as an electrolyte solution for a lithium secondary battery having high energy density, there have been used electrolyte solutions prepared by dissolving a lithium electrolyte into carbonates having high withstand voltage such as dimethyl carbonate, ethylene carbonate and the like. However, since such carbonates are flammable liquid, various countermeasures for securing safety have been required. Further, the development of an electrolyte solution having higher safety has been eagerly desired recently wherein the development of a large-scale lithium secondary battery is in progress.
For improving safety of such an electrolyte solution, there is, for example, a suggestion in which the inflammability of an electrolyte is enhanced by using phosphates such as trimethyl phosphate, triethyl phosphate and the like as the non-aqueous solvent for the electrolyte solution (Japanese Patent Application Laid-Open (JP-A) No. 4-184870). However, since use of the above-described phosphates causes reduction in battery capacity, the development particularly of a large-scale lithium secondary battery is problematical.
The object of the present invention is to solve the above-described problems and to provide a non-aqueous electrolyte solution which has high energy density when used in a lithium secondary battery and further has excellent safety and a lithium secondary battery using this solution.
The present inventors have intensively studied for attaining the above-described object, and found that a non-aqueous electrolyte solution obtained by adding a phosphoric acid compound having a specific group is a non-aqueous electrolyte solution having high safety of battery and high energy density simultaneously since the solution has further excellent safety and small reduction in capacity when used in a battery, and have completed the present invention.
Namely, the present invention relates to (1) a non-aqueous electrolyte solution comprising a non-aqueous solvent and a lithium electrolyte, wherein the non-aqueous solvent is obtained by adding a phosphoric acid compound prepared by substituting at least one hydrogen atom in phosphoric acid or polyphosphoric acid with a group represented by the general formula [1]: (hereinafter, the phosphoric acid compound prepared by substituting at least one hydrogen atom in phosphoric acid with a group represented by the general formula [1] may be referred to as xe2x80x9cphosphate compoundxe2x80x9d, and the polyphosphoric acid compound prepared by substituting at least one hydrogen atom in polyphosphoric acid with a group represented by the general formula [1] may be referred to as xe2x80x9cpolyphosphate compoundxe2x80x9d.) 
(wherein, X represents Si, Ge or Sn atom, and each of R1 to R3 independently represents an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms or an aryl group having 6 to 10 carbon atoms.).
Also, the present invention relates to (2) a non-aqueous electrolyte solution comprising a non-aqueous solvent and a lithium electrolyte, wherein the non-aqueous solvent contains a phosphoric acid compound prepared by substituting at least one hydrogen atom in phosphoric acid or polyphosphoric acid with a group represented by the general formula [1].
Further, the present invention relates to (3) a lithium secondary battery comprising a cathode which can be doped or undoped with a lithium ion, an anode comprising a lithium metal or lithium alloy, or an anode which can be doped or undoped with a lithium ion, a non-aqueous electrolyte solution and a separator, wherein the non-aqueous electrolyte solution is a non-aqueous electrolyte solution according to (1) or (2).
Furthermore, the present invention relates to (4) the lithium secondary battery according to (3), wherein the cathode which can be doped or undoped with a lithium ion comprises a lithium composite oxide containing at least one of manganese, iron, cobalt or nickel.
Moreover, the present invention relate to (5) the lithium secondary battery according to (3), wherein the anode which can be doped or undoped with a lithium ion comprises at least one carbon material selected from the group consisting of natural graphite, artificial graphite and coke.